A number of combustion processes for a furnace use a burner supplied with air as an oxidizer in combination with a fuel, such as natural gas, fuel oil, propane, waste oils, other hydrocarbons, and the like. Attempts have been made to improve the performance of such air combustion processes by enriching the combustion atmosphere with oxygen enriched air, or pure oxygen gas. Oxygen enrichment of the combustion air increases both the burner flame temperature and the thermal efficiency while the furnace flue gas volume decreases as the oxygen concentration in the air or oxidizing gas increases.
It is known that even low level oxygen enrichment in the combustion process can cause a dramatic increase in undesirable nitric oxide (NO.sub.x) emissions. In industrial combustion processes, over 90% of the NO.sub.x emissions are in the form of nitric oxide or NO. High levels of oxygen enrichment, e.g., above 90% total oxygen content in the oxidizer, could result in the production of less NO.sub.x than using air for the same burner firing rate. However, high levels of oxygen enrichment are costly to implement.
Further, when oxygen is used to replace the air for combustion, it often causes problems, such as furnace refractory damage, uneven temperature distribution, and high NO.sub.x emission due to high flame temperature. In specialized applications of metal processing, especially in aluminum remelting, another related problem occurs, namely excess oxidation of the metal load.
Conventionally, one approach used to enrich the oxygen content of the combustion process is to install an oxy-fuel burner in the center of the existing air-fuel burner. This has a disadvantage in that it results in a relatively complex construction. Further, in such a burner it is difficult to control the two fuel streams and, at the same time, to adjust both the air and the oxygen for matching the fuel streams. Another approach is to design an oxy-fuel burner which can utilize a high level of oxygen as an oxidant and yet still maintain a moderate flame temperature and low NO.sub.x emissions. This involves a new burner installation involving more work which can be difficult and costly.
Accordingly, a need exists to develop a system as a retrofit to an existing air burner system to enable the use of both oxygen and air for combustion without causing the undesired adverse affects associated with using only pure oxygen as the oxidant.